Ultrasonic motion detectors project and receive ultrasonic sound energy in a region of interest. Object motion within the region of interest and in the range of the ultrasonic motion sensor is detected and an alarm signal representative thereof is produced. The actual or effective range of ultrasonic motion detectors, however, differs from design range whenever the actual ambient atmospheric sound propogation conditions vary from the design or nominal atmospheric conditions. False alarms are produced should the ambient atmospheric conditions be such as to provide an effective range that is greater in spatial extention than the design range. In this case, object motion is detected that arises beyond the region of interest. A failure of alarm situation occurs should the ambient atmospheric conditions be such as to produce an effective range that is spatially less extended than the design range. In this case, object motion within the region of interest, but beyond the actual range of the detector, goes undetected.